Evolution And History

288 CHAPTER 12 | Modern Human Diversity: Race and Racism

ranking human beings with respect to their intelligence

scores in terms of “racial” difference is doubly false.

Over the past 2.5 million years, all populations of the

genus Homo have adapted primarily through culture—

actively inventing solutions to the problems of survival,

rather than relying only on biological adaptation. Thus

we would expect a comparable degree of intelligence in all

present-day human populations. The only way for indi-

vidual human beings to develop their innate abilities and

skills to the fullest is to ensure that they have access to the

necessary resources and the opportunity to do so.

Studying Human

Biological Diversity

Although the biological category of race is not valid

when considering groups of humans, this is not to say

that differences in various biological traits such as skin

color do not exist. In fact, skin color provides an excel-

lent example of the role of natural selection in shaping

human variation.

The physical characteristics of both populations and

individuals are a product of the interaction between

genes and environments. For example, genes predispose

people to a particular skin color, but an individual’s skin

color is also influenced by cultural and environmental

factors. The skin of sailors, for example, is darkened or

burned after many hours of exposure to the sun, de-

pending on not only genetic predisposition but cultural

practices regarding exposure to the sun. In other cases,

such as A-B-O blood type, phenotypic expression closely

reflects genotype.

For characteristics controlled by a single gene, differ-

ent versions of that gene, known as alleles (see Chapter 2),

also mediate variation. Such traits are called polymorphic

(meaning “many shapes”). Our blood types—determined

by the alleles for types A, B, and O blood—are an example

of a polymorphism and may appear in any of four distinct

pheno typic forms (A, B, O, and AB).

A species can also be considered polymorphic, mean-

ing that there is wide variation among individuals (beyond

differences between males and females). Here “polymor-

phic” refers to continuous phenotypic variation that may

be genetically controlled by interactions among multiple

different genes, in addition to the allelic variation de-

scribed above. When a polymorphic species faces chang-

ing environmental conditions, the variation it has within

its gene pool fosters survival of the species, since some of

those individuals may possess traits that prove adaptive in

the altered environment. Individuals whose physical char-

acteristics enable them to do well in the new environment

will usually reproduce more successfully, so that their

It is also well known that IQ scores rise in proportion to

the test-takers’ amount of schooling.

More such cases could be cited, but these three obser-

vations suffice to make the point. First, there is a bias in IQ

testing based on social class. Second, the assertion that IQ

is biologically fixed and immutable is clearly false. Third,

Differences in the growth process can lead to very different outcomes

in terms of size as seen in these twins, who are genetically identical.

Even starting from inside their mother’s womb, twins may experience

environmental differences in terms of blood and nutrient supply. This

can impact not only size but cognitive development.

From Komai, T., & Fukuoka, G. (1934, October). Post-natal growth disparity in monozygotic twins.

Journal of Heredity

25

, 423–430.

polymorphic Describing species with alternative forms

(alleles) of particular genes.